first pass experiment to try alternate tone spacings, works quite well

git-svn-id: https://svn.code.sf.net/p/freetel/code@3445 01035d8c-6547-0410-b346-abe4f91aad63

diff --git a/codec2-dev/octave/fsk_basic_alt.m b/codec2-dev/octave/fsk_basic_alt.m
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+++ b/codec2-dev/octave/fsk_basic_alt.m
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+% fsk_basic_alt.m
+% David Rowe April 2018
+%
+% Modification of fsk_basic.m, to explore idea of alternating tone frequencies
+% for multipath channels.
+
+#{
+  [ ] IL curves for half tone spacing
+      + cf to channel with multipath
+#}
+
+% make sure we get same results from random number generators on each run
+
+rand('seed',1);
+randn('seed',1);
+
+% constants -----------------------------------------------------------
+
+Fs = 8000;     % sample rate
+f1 = 1000;
+f2 = 1800;
+f3 = 1400;
+f4 = 2200;
+Rs = 800;      % symbol rate
+Ts = Fs/Rs;    % length of each symbol in samples
+
+% simulation parameters -----------------------------------------------
+
+Nbits = 10000;
+EbNodB = 9;    
+
+% start simulation ----------------------------------------------------
+
+tx_bits = round(rand(1,Nbits));
+
+% continuous phase FSK modulator
+
+w1 = 2*pi*f1/Fs;
+w2 = 2*pi*f2/Fs;
+w3 = 2*pi*f3/Fs;
+w4 = 2*pi*f4/Fs;
+tx_phase = 0;
+tx = zeros(1,Ts*Nbits);
+
+for i=1:Nbits
+  for k=1:Ts
+    if mod(i,2)
+      if tx_bits(i)
+        tx_phase += w2;
+      else
+        tx_phase += w1;
+      end
+    else
+      if tx_bits(i)
+        tx_phase += w4;
+      else
+        tx_phase += w3;
+      end
+    end    
+    tx((i-1)*Ts+k) = exp(j*tx_phase);
+  end
+end
+
+% AWGN channel noise
+
+EbNo = 10^(EbNodB/10);
+variance = Fs/(Rs*EbNo);
+noise = sqrt(variance/2)*(randn(1,Nbits*Ts) + j*randn(1,Nbits*Ts));
+
+d = Ts/2; l = length(tx);
+rx = tx + noise;
+
+figure(1); clf;
+Tx = abs(fft(tx(1:Fs)));
+plot(Tx);
+
+% integrate and dump demodulator
+
+rx_bits = zeros(1,Nbits);
+for i=1:Nbits
+  arx_symb = rx((i-1)*Ts + (1:Ts));
+  if mod(i,2)
+    filt1 = sum(exp(-j*w1*(1:Ts)) .* arx_symb);
+    filt2 = sum(exp(-j*w2*(1:Ts)) .* arx_symb);
+  else
+    filt1 = sum(exp(-j*w3*(1:Ts)) .* arx_symb);
+    filt2 = sum(exp(-j*w4*(1:Ts)) .* arx_symb);
+  end
+  rx_bits(i) = filt2 > filt1;
+end
+
+Nerrors = sum(xor(tx_bits, rx_bits));
+ber = Nerrors/Nbits;
+printf("EbNodB: %4.1f  Nerrors: %d BER: %1.3f\n", EbNodB, Nerrors, ber);
+